Is my fish oil rancid?

First off, there is a lot of misinformation here. Just because fish oil smells does NOT mean it is oxidized, or what everyone is calling rancid. Fish oil smells like fish, and certain species of fish have more or less smell than others. Also, just because a fish oil doesn't smell does NOT mean it hasn't oxidized, either. You can use deodorizing methods to get rid of the smell, but actually have the oil be oxidized. The only way to empirically measure if fish oil is oxidized is doing a peroxide test. Different species of fish can smell and taste totally different. One fish oil with a 2 mEq/Kg peroxide value could taste totally different than another oil made from a different species, even if that one also has 2 mEq/Kg. A good limit for peroxide levels in fish oil is 5 mEq/Kg or less. What people traditionally consider the rancid taste starts happening in the 30-40 mEq/Kg range. However, remember that deodorization techniques can mask that sometimes. As always, it is much more complex than the internet would have you believe. Ask for the measured peroxide values of that batch from the vendor you bought from.